Soft magnetic materials fabricated by powder metallurgy are used in electric appliances having a solenoid valve, motor, electric circuit, or the like. The soft magnetic material is composed of a plurality of composite magnetic particles, and the composite magnetic particles have metallic magnetic particles composed of pure iron, for example, and an insulation film composed of phosphate, for example, that covers the surface of the particles. Based on a need to improve the energy conversion efficiency, reduce heat output, and achieve other needs in a soft magnetic material, a magnetic property is required that allows a considerable magnetic flux density to be obtained by the application of a weak magnetic field, and a magnetic property is required in which energy loss is low in magnetic flux density fluctuations.
When a powder magnetic core fabricated using this soft magnetic material is used in an alternating magnetic field, energy loss occurs that is referred to as “iron loss.” This iron loss is expressed as the sum of hysteresis loss and eddy current loss. Hysteresis loss is a loss of the energy required for varying the magnetic flux density of the soft magnetic material. Eddy current loss is an energy loss produced by eddy current that flows between the metallic magnetic particles constituting the soft magnetic material. Hysteresis loss is proportional to the operating frequency, and eddy current loss is proportional to the square of the operating frequency. For this reason, hysteresis loss is primarily dominant in a low-frequency range, and eddy current loss is dominant in a high-frequency range. A powder magnetic core must have magnetic characteristics that correspond to the generation of minimal iron loss, i.e., high alternating-current magnetic characteristics.
Magnetic domain walls can be moved more easily in order to reduce hysteresis loss in particular among the types of iron loss of a powder magnetic core, and the coercive force Hc of the metallic magnetic particles can be reduced to achieve this. In view of the above, pure iron, which is a material that has a low coercive force Hc, has conventionally been used on a wide scale for metallic magnetic particles. A technique for reducing hysteresis loss is disclosed, for example, in Japanese Laid-Open Patent Application No. 2005-15914 (Patent Document 1), wherein the mass ratio of impurities with respect to the metallic magnetic particles is set to 120 ppm or less by using pure iron as the metallic magnetic particles.
There is also a method for reducing hysteresis loss of the powder-magnetic core in which the metallic magnetic particles are heat-treated before an insulation layer is formed, or the molded article is heat-treated after pressure molding. Using these heat treatments, strain, grain boundaries, and the like present in the metallic magnetic particles can be removed, magnetic domain walls can be moved more easily, and the coercive force Hc of the metallic magnetic particles constituting the soft magnetic material can be reduced. Japanese Laid-open Patent Application 2002-246219 (Patent Document 2), for example, discloses a technique for heating a pressure-molded article in air for 1 hour at a temperature of 320° C. and then further heating the article for 1 hour at a temperature of 240° C.
Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2005-15914
Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2002-246219